Measurement of the Adsorption Isotherm

This is by far the easiest parameter to obtain. The polymeric surfactant concentration is measured before Cj) and after (Qq iHbrium. 2)  

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In the measurement of the adsorption isotherm, r=f(p)T A, the variables T ar are held constant. If we wish to obtain the analytical relation between r and p f Equation (2.20), then it is necessary to find another adsorption potential. For purpose we must characterize the adsorbed phase by another intensive variable. 

Direct determination of the column saturation capacity requires measurement of the adsorption isotherm. Use of methods such as frontal analysis, elution by characteristic point are classical techniques. Frontal analysis and elution by characteri.stic point require mg or gram quantities of pure product component. It is also possible to estimate the column saturation capacity from single-component overloaded elution profiles using the retention time method or using an iterative numerical method from a binary mixture [66J. 

Volumetric measurements of the adsorption isotherms of hydrogen on SWNTs show that they are type 1 reversible isotherms similar to activated carbons (Fig. 10.7). Although a substantial dispersion in the experimental values of the adsorbed density was observed, improved measurements have been shown to be reliable. The small amount of SWNTs typically available for adsorption experiments requires that adsorbed density measurements be performed with sensitive instruments. The purity of the hydrogen used in the sorption experiments can also be an issue. In addition, the synthesis and preparation of the samples can influence substantially adsorption on SWNTs. Notably, thermal treatments can enhance the adsorption properties of S WNTs by removing guest species blocking access to adsorption sites. 

This calculation method is often simpler, because it does not require the measurements of the adsorption isotherm parameters in the region of low surfactant concentrations, while the measurements of surface and interfacial tension in the concentration range close to the CMC are experimentally much easier performed. 

Szepesy, L. Hies, V Adsorption of Gases and Gas Mixtures, II. Measurement of the Adsorption Isotherms of Gases on Active Carbon under pressures of 1 to 7 atm, Acta Chim. Hung. 35 (1963), p. 53/60... 

Nitrogen adsorption Measurement of the adsorption isotherm of a porous material at liquid nitrogen temperature. The monolayer coverage by nitrogen measures the specific surface area... 

The differential molar enthalpy of displacement can be approximated by the iso-steric heat of adsorption, As,fti. This indirect (noncalorimetric) method is based on measurement of the adsorption isotherms at neighboring temperatures. For... 

For most adsorption experiments the temperature at which the measurements are made is less than the triple point of the gas being used but above its freezing point. This being the case, one would normally expect that the adsorbate characteristics resemble the liquid phase rather than the solid phase of the adsorptive. This is the normal assumption used for most adsorption theories. The principle measurement performed as an adsorption experiment is the measurement of the adsorption isotherm. The adsorption isotherm is the measurement of amount adsorbed versus adsorptive pressure at constant temperature. This is the easiest measurement to make. Another type of measurement is calorimetry. One form of calorimetry measures the amount of heat evolved as the adsorptive is adsorbed. Another form measures the heat capacity of the adsorbate. There are various forms of calorimetry but the most accurate methods are very difficult to perform and only a few examples are available in the literature. Another form of calorimetry, which is easier to perform, is scanning calorimetry. This calorimetry form is a good tool to determine qualitative features of the adsorption and to yield a fair indication of the physical quantities. 

It is widely accepted that the knowledge of adsorption heats is vital in the description of gas-solid interaction. This is particularly useful when adsorption heat measurements are combined with simultaneous measurement of the adsorption isotherm. These measurements obviously may provide information about the energetic of surface processes. In some simple cases, even information on the structure of the surface itself, like for example the energetic topography, can be retrieved from adsorption heats and isotherms [1,2]. Chemisorption and catalyzed reactions, like any chemical reaction, are associated with changes of enthalpy and can therefore be studied by means of calorimeters. Many calorimeters, operating on different principles, have indeed been used for this purpose [3—5]. Adsorption calorimeters are particularly convenient for these studies [4]. They offer a number of advantages which will be illustrated by means of selected examples. 

Following the pioneer work of Beebe in 1945, the adsorption of krypton at 77 K has come into widespread use for the determination of relatively small surface areas because its saturation vapour pressure is rather low (p° 2Torr). Consequently the dead space correction for unadsorbed gas is small enough to permit the measurement of quite small adsorption with reasonable precision. Estimates of specific surface as low as 10 cm g" have been reported. Unfortunately, however, there are some complications in the interpretation of the adsorption isotherm. 

Determination of the equilibrium spreading pressure generally requires measurement and integration of the adsorption isotherm for the adhesive vapors on the adherend from zero coverage to saturation, in accord with the Gibbs adsorption equation [20] ... 

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

In this paper we present results for a series of PEO fractions physically adsorbed on per-deutero polystyrene latex (PSL) in the plateau region of the adsorption isotherm. Hydro-dynamic and adsorption measurements have also been made on this system. Using a porous layer theory developed recently by Cohen Stuart (10) we have calculated the hydrodynamic thickness of these adsorbed polymers directly from the experimental density profiles. The results are then compared with model calculations based on density profiles obtained from the Scheutjens and Fleer (SF) layer model of polymer adsorption (11). 

The precise measurement of competitive adsorption isotherms not only of theoretical importance but may help the optimization of chromatographic processes in both analytical and preparative separation modes. The methods applied for the experimental determination of such isotherms have been recently reviewed [90], Frontal analysis using various flow rates can be successfully applied for the determination of competitive adsorption isotherms [91]. 

Fig. 32 Schematic representation of the correlation of surface charge and growth of the aggregates for the various regions of the adsorption isotherm of SDS on alumina based on fluorescence data and zeta potential measurement...

When a surfactant is injected into the liquid beneath an insoluble monolayer, surfactant molecules may adsorb at the surface, penetrating between the monolayer molecules. However it is difficult to determine the extent of this penetration. In principle, equilibrium penetration is described by the Gibbs equation, but the practical application of this equation is complicated by the need to evaluate the dependence of the activity of monolayer substance on surface pressure. There have been several approaches to this problem. In this paper, previously published surface pressure-area Isotherms for cholesterol monolayers on solutions of hexadecy1-trimethyl-ammonium bromide have been analysed by three different methods and the results compared. For this system there is no significant difference between the adsorption calculated by the equation of Pethica and that from the procedure of Alexander and Barnes, but analysis by the method of Motomura, et al. gives results which differ considerably. These differences indicate that an independent experimental measurement of the adsorption should be capable of discriminating between the Motomura method and the other two. 

Figure 6.2,15 shows the adsorption isotherms of C02 on the CaHAP particles with different Ca/P ratios (67). Before the measurement of the first isotherms, shown by the open symbols, the samples were outgassed at 300°C for 2 h. The second set of isotherms, shown by the solid symbols, were measured on the samples outgassed at 25°C for 2 h after taking the first isotherms. The first and second isotherm sets are parallel, and the adsorbed amount of the second isotherm is less than that of the first one, which signifies that a part of the C02 adsorbed is irreversibly adsorbed. The amount of the irreversibly adsorbed C02 (noted as n,) was evaluated by subtracting the adsorbed amount at 500 torr in the second isotherm from that in the first one. The values are plotted against Ca/P ratios by the open symbols in Figure 6.2.16. Similar results were reported on SrCaHAP (29), SrHAP (68), and MgCaHAP...

Chlorofluorocarbon (CFC) replacements have recently been used for their lower stability and because they have carbon-hydrogen bonds, which means that their atmospheric lifetime is expected to be much shorter than those of CFCs. The adsorption properties of l,l,2-trichloro-l,2,2-trifluoroethane (CFC 113) and its replacement compounds, l,l-dichloro-2,2,2-trifluoroethane (HCFC123), 1,1-dichloro-l-fluoroethane (HCFC141b), and l,l-dichloro-l,2,2,3,3-pentafluoropropane (HCFC225ca) on four kinds of activated carbons were investigated. The adsorption isotherms of inhalational anesthetics (halothane, chloroform, enflurane, isoflurane, and methoxyflurane) on the activated carbon were measured to evaluate the action mechanism of inhalational anesthesia. The anesthesia of CFC replacements can be estimated by the Freundlich constant N of the adsorption isotherms (Tanada et al., 1997). 

Typical of the sort of data needed to determine whether additives affect the interface is that provided by a study of the influence of n-heptyl compounds on the gel structure of dispersions containing polar solids in nonpolar vehicles (70). The influence of the polar heptyl compounds on the fluidity of dispersions of rutile and a fine silica (HiSil) in a dibasic ester, Plexol 201, is shown in Fig. 7. Apparently, the more polar rutile adsorbs all except the chloride and in these cases thinning results. HiSil has a lower F value and adsorbs only the amine and alcohol preferentially. Greases prepared from the least polar solid, Aerosil, are also least influenced by these additives (or even by more complex ones). Measurements of the solution isotherms for HiSil and Aerosil reveal significant adsorption of heptyl alcohol, but no detectable chloride adsorption in the same concentration range. 

We have also compared the determination of the adsorption isotherms with those from a standard gravimetric technique. The precision of the data obtained by the gravimetric technique was found to be lower than that obtained by the FT-IR technique, particularly at very low silane concentrations (about 5%, and 3% at 0.4 g/100 ml concentration for the gravimetric and the FT-IR techniques, respectively). This is because very small weights of the silanes cannot be measured accurately and the silane is volatile. The effects of suspended silica particles in the solution were also examined by centrifuging methanol solutions... 

There is a fundamental relationship described in chromatographic theory between the retention volume of a elution peak and the mid-point of a breakthrough curve achieved by operating the column under frontal analysis conditions (41 ). In the Henry s Law region of the adsorption isotherm, the net retention volume and its measurement can be used to describe the variation of sorbate breakthrough volume as illustrated in Figure 8. Utilizing the experimental apparatus described in the last section, retention volumes were measured as a function of pressure at 40°C (T =... 

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